1. Field of Invention
This invention relates to a digital rod position indication system for use in a nuclear power plant. More specifically, this invention relates to a digital rod position indication system capable of monitoring rod drop times during normal plant operation.
2. Description of the Related Art
In a Pressurized Water Reactor (PWR), the power level of the reactor 10 is controlled by inserting and retracting the control rods 12, which for purposes of this application include the shutdown rods, into the reactor core 14. The control rods are moved by the Control Rod Drive Mechanisms (CRDM), which are electromechanical jacks that raise or lower the control rods in increments. The CRDM includes a lift coil DML, a moveable gripper coil DMM, and a stationary gripper coil DMS that are controlled by the Rod Control System (RCS) and a ferromagnetic drive rod that is coupled to the control rod and moves within the pressure housing 16. The drive rod includes a number of circumferential grooves at ⅝ inch intervals (“steps”) that define the range of movement for the control rod. A typical drive rod contains approximately 231 grooves, although this number may vary. The moveable gripper coil mechanically engages the grooves of the drive rod when energized and disengages from the drive rod when de-energized. Energizing the lift coil raises the moveable gripper coil (and the control rod if the moveable gripper coil is energized) by one step. Energizing the moveable gripper coil and de-energizing the lift coil moves the control rod down one step. Similarly, when energized, the stationary gripper coil engages the drive rod to maintain the position of the control rod and, when de-energized, disengages from the drive rod to allow the control rod to move. The RCS includes the logic cabinet and the power cabinet. The logic cabinet receives manual demand signals from an operator or automatic demand signals from Reactor Control and provides the command signals needed to operate the shutdown and control rods according to a predetermined schedule. The power cabinet provides the programmed dc current to the operating coils of the CRDM.
Current PWR designs have no direct indication of the actual position of each control rod. Instead, step counters associated with the control rods are maintained by the RCS and rod position indication (RPI) systems to monitor the positions of the control rods within the reactor. The associated step counter is incremented or decremented when movement of a control rod is demanded and successful movement is verified. Because the step counter only reports the expected position of the control rod, certain conditions can result in the step counter failing and deviating from the actual position of the control rod. In certain situations where the actual position of the control rod is known, the step counter can be manually adjusted to reflect the actual position. However, if the actual position of the control rod is not known, a plant shutdown may be required so that the step counters to be initialized to zero while the control rods are at core bottom.
The RPI systems derive the axial positions of the control rods by direct measurement of drive rod positions. Currently both analog rod position indication (ARPI) systems and digital rod position indication (DRPI) systems are in use in PWRs. The conventional DRPI systems have been in service for over 30 years in nuclear power stations worldwide and are currently being used as the basis for the rod position indication systems in the new Westinghouse AP1000 designs.
A conventional DRPI system includes two coil stacks for each control rod and the associated DPRI electronics for processing the signals from the coil stacks. Each coil stack is an independent channel of coils placed over the pressure housing. Each channel includes 21 coils. The coils are interleaved and positioned at 3.75 inch intervals (6 steps). The DRPI electronics for each coil stack of each control rod are located in a pair of redundant data cabinets (Data Cabinets A and B). Although intended to provide independent verification of the control rod position, conventional DRPI systems are not accurate to fewer than 6 steps. The overall accuracy of a DRPI system is considered to be accurate within ±3.75 inches (6 steps) with both channels functioning and ±7.5 inches using a single channel (12 steps). In contrast to the conventional DRPI system, a conventional ARPI system determines the position based on the amplitude of the dc output voltage of an electrical coil stack linear variable differential transformer. The overall accuracy of a properly calibrated ARPI system is considered to be accurate within ±7.2 inches (12 steps). Neither conventional ARPI systems nor conventional DRPI systems are capable of determining the actual positions of the control rods.
It should be noted that for purposes of this application, the phrase “control rod” is used generically to refer to a unit for which separate axial position information is maintained, such as a group of control rods physically connected in a cluster assembly. The number of control rods varies according to the plant design. For example, a typical four-loop PWR has 53 control rods. Each control rod requires its own sets of coils having one or more channels and the DRPI electronics associated with each channel. Thus, in a typical four-loop PWR, the entire DPRI system would include 53 coil stacks, each having two independent channels, and 106 DPRI electronics units. Further, in this application, the phrase “coil stack” is used generically to refer to the detector coils associated with each control rod and should be understood to include either or both channels of detector coils. Thus, a measurement across a coil stack contemplates the value across both channels combined and/or the value across a single channel.
Unfortunately, aging and obsolescence issues have led to an increase in problems with conventional DRPI systems including analog card failures and coil cable connection problems that, in some cases, may result in unplanned reactor trips. These problems, along with plans for plant life extension, have prompted the industry to actively seek viable options to monitor the health and accuracy of the DRPI systems and/or to replace failing systems in order to ensure reliable plant operations for decades to come.
Beyond the technical problems of the conventional DRPI systems, regulatory issues exist. Many existing PWRs are approaching the end of qualified life for several components of the conventional DRPI systems during the next decade and are actively seeking replacement options at this time. There has been a significant push in recent years for plants to replace aging analog systems with digital systems made from commercially-available off-the-shelf parts. Using readily-available commercial parts provide plants more options for replacement in the future.
One concern for PWRs is ensuring that rod drop times meet regulation. In the event of a reactor trip, both the moveable gripper coil DMM and the stationary gripper coil DMS are de-energized releasing the control rods to gravity. The maximum allowable time for all rods to reach core bottom is prescribed by regulation and a PWR must periodically test the rod time to ensure compliance. Conventional rod drop testing occurs outside of normal plant operation with the rod drop being initiated by plant operators. Thus, in conventional rod drop testing, the start time is planned and, therefore, known.
One method for performing a conventional rod drop test involves fully withdrawing the control rods, deactivating the DRPI system, and releasing the control rods. As the ferromagnetic drive rods pass through the de-energized detector coils, a signal proportional to the velocity of the control rod is generated in each detector coil due to the magnetic field effects. These outputs are used to determine the rod drop time.
More recently, the assignee of the present invention has offered online rod drop testing, as disclosed in U.S. Pat. No. 6,404,835 (the '835 patent). The method of rod drop testing is accomplished by attaching a data acquisition unit to the reference test point PTREF of each data cabinet in order to sample the analog signal across the DRPI coil stack. The sampled analog data is then sent to a testing computer located outside containment at the rod control power cabinets. A filter applied to the sampled analog data removes the applied coil power and leaves the composite induced signal resulting from the drive rod passing through the coils (i.e., the rod drop trace) appearing at the reference test point. While this method of testing rod drop times does not require the plant to be taken offline as with the previous method, the rod drop tests occur outside of the normal plant operation. Thus, in the event of an actual reactor trip, there is no way to verify compliance with the regulation drop time limits.